The present invention relates to a method based on a novel control and steering strategy for use in the drying process of a paper web or similar coated web material such as board in coater sections in which the web to be coated is passed via a coater station or applicator section including at least one applicator apparatus, dryers and a calendar.
The invention also relates to a method for controlling web moisture during on-line calendering, particularly supercalendering, in conjunction with the drying process of a papermaking machine in the manufacture of uncoated paper grades.
In the coating of a web of paper or board, the surface of the web is first coated with a furnish containing coating pigments slurried in water. After the application and smoothing of the coating mix, the coating applied to the web surface as well as the underlying base web must be dried to a sufficiently low moisture for final use or further processing. Subsequent to the coating process, the sheet can be calendered in different ways using, e.g., a machine calender, a soft-nip calender, a super-calender or other type of multinip calender. In all units of these calender variants, the sheet gloss and smoothness are improved utilizing heat, moisture control and varying nip pressure in combination. Hence, a major portion of the energy consumed in the production of coated paper grades is lost in drying the web during the different steps of postprocessing, which means that energy management in drying is an extremely vital factor contributing to the profitability of production. Correct drying technique also affects the quality of the produced paper grade. Another parameter highly pertinent to the quality of produced paper is the control of the machine-direction moisture profile, that is, the moisture of the base paper, which must be kept at a constant level during the run. The web moisture content affects particularly the paper web behavior in calendering and printing. As modern production lines are equipped with on-line calendering, wherein the coated web is passed directly to a calender, the moisture profile of the running web has an insufficient time to reach a uniform equilibrium state prior to calendering, a situation which is in contrast to that attainable in the traditional off-line calendering, wherein the coated web was stored in a machine reel prior to subsequent calendering. Correspondingly, the transport chain of paper from the mill to printing houses and other users has been speeded up, whereby the moisture even in uncalendered paper does not necessarily have enough time to stabilize and reach a sufficiently low level prior to printing. In coating, the web moisture content affects the penetration of water into the base web during the application of the coating mix and, resultingly, the change of coating solids content after coating. As variations in the solids content of the coating are reflected in plural parameters in the application process, it is important to keep the web moisture during application and drying accurately within proper limit values in order to attain a uniform and desired final quality of the product.
Correct web moisture is particularly important in calendering and, moreover, in supercalendering. Usually, the uncoated or coated paper web entering the calender is too dry for direct calendering. As a web of higher moisture is easier to calender than a web of lower moisture, the web is moistened at least in supercalendering by means of steam jets even in several stages during calendering in order to reach the optimum result from multistage calendering in terms of web surface quality and strength.
Conventionally, a coated web is dried immediately after the application of coating using noncontacting dryers, which step may be followed when necessary by cylinder dryers and other dryers of the contacting type. The moisture content of the running web is measured at multiple points along the web travel in the coater apparatus and, on the basis of the measurement data, the drying effect of each dryer is individually adjusted so as to attain a proper web moisture over the cross-machine width at the respective measurement point as well as an average moisture content that stays between given limits during a run, the latter requirement meaning that the machine-direction moisture profile is controlled to a given set value. The overall drying capacity is adjusted to a suitable basic level based on test runs and data accumulated from a long-term experience in the art, and the individual dryer effects are then fine-tuned during the run on the basis of measurement data either automatically or manually. Conventionally, one of the dryers or one dryer group is selected to perform as the controller of the final moisture level, whereby the heating power input to the selected dryer group(s) is adjusted by means of a feedback signal obtained from the measurement system. In this arrangement, the other dryers are driven under manual control. Such a control scheme responds very tardy and compensation for the slow response of dryer control is difficult to implement in situations requiring a fast change of dryer effect levels. Furthermore, the web temperature prior to the coater apparatus must be kept sufficiently low to avoid floccing of the coating mix being applied. Hence, proper control of the drying effect is important particularly in the final stage of the dryer section prior to the subsequent coating step. The web temperature also affects the final quality of the coated web.
As concurrent methods are awkward to use in the control of the drying process so that the web in each stage could be kept at an accurately optimized moisture, it has been customary, particularly in off-line supercalendering, to first bring the dried web down to a minimum moisture, whereupon it is rewetted to a suitable moisture for calendering. In the nips of the calender rolls, the excess moisture absorbed into the web from steam treatment is removed, but even here it is difficult to control the moisture in the end product as well as in its intermediate stages, because moisture variations as small as a few per cent already will affect the end product quality. If deviations in the initial moisture content of the web are allowed, these will be reflected in the quality of the end product notwithstanding the situation that measurements after the calendering step would indicate a desired value of the end product moisture.
Particularly in situations of changing running conditions or when starting up the machine, known in the art as the run-up, the elevation of the dryer drying effect levels to correct values and adjustment of the same to proper run-time levels requires excellent skills from the personnel operating the machine. However, carrying out the procedure of setting the dryer evaporation effect levels in the coater section and the calender steaming rates and roll temperatures to correct values under run-up or changing process conditions takes time, during which the produced paper or board falls short of the specified quality requirements thus necessitating dumping of the web into the pulper. Hence, it is advantageous to minimize the durations of run-up and process value change times in order to achieve improved production efficiency at the machine. The above control scheme is also extremely clumsy in the optimization of drying energy consumption inasmuch it relies on the control of each dryer unit separately, whereby the mutual evaporation effect ratios between the dryer units are difficult to alter in an uncomplicated manner. Furthermore, a failure in one or a greater number of the dryer units is difficult to compensate for, because the process is designed for operation with all the dryer units being functional.
It is another object of the present invention to provide a method suited for controlling the machine direction moisture profile of a web to be calendered or to be coated and calendered in a manner optimized to respond to any moisture changes throughout the entire coating/drying process. In practice this approach means the application of a comprehensive control scheme covering all the dryer units and calender of a coater section in an integrated manner in regard to energy consumption and product quality in order to attain an optimal end result.
The goal of the invention is achieved by way of forming a mathematical submodel of specific moisture evaporation rate for each process section and device contributing to the web drying process and then chaining the thus obtained individual submodels so as to form a composite model of the overall process, the model being suited for managing the drying phenomena during the entire process so that each individual unit of the equipment layout is controlled as a part of the overall process. In its simplest form, the overall process model is formed by a submodel of the calender and a submodel that at least partially characterizes the unit preceding the calender.
The invention offers significant benefits.
By virtue of the model according to the invention, it is possible to directly compute the moisture content of the web at the outgoing side or each dryer and calender nip, provided that the specific evaporation rate at the dryer or the nip, as well as at an associated open draw, and the web moisture at the ingoing side are known. After the chaining of the individual submodels, the web moisture content can be computed at different points along the machine, the most important parameter value obviously being the final moisture content of the web. With the help of the model, the dryer effects may be adjusted according to the individual properties so that the characteristics of different types of dryers are optimally taken into account. Since infrared dryers feature a quick response, they may be used, e.g., during run-up for controlling the overall effect of the dryer group, thus allowing the evaporation effect levels of other dryers to be elevated in a more relaxed manner to their steady-state values during the normal run by way of compensating for the delay of dryer warm-up with the help of delay terms adapted into the model. The use of delay terms makes it possible to manage actual process response delays, too.
Since the invention provides a control scheme for the overall process, it allows the evaporation effects of the dryer units to be divided therebetween in a desired manner and, particularly in the case of failure in one dryer, the drying effect lost thereby may be compensated for by the other dryer units thus permitting operation of the coater section uninterrupted by a servicing shutdown. Equally, as the initial moisture content of the web as well as the amount of moisture added thereto by the applied coating or through wetting are known, the model gives tools for computing an estimate for the web moisture at different points along the process and, particularly, prior to upwinding. In fact, the model allows the web final moisture content to be computed so accurately that production may be continued controlled by the model even when the moisture measurement devices are down.
The overall performance offered by the invention gives a faster and more accurate control result than that available by way of manual control combined with feedback loops controlling the individual drying units.